Theoretical Analysis And Experiments Of Ship Drag Reduction By Microbubble

Because microbubble could effectively reduce the friction resistance between solid wall and other medium, more and more scholars contribute themselves to it. The resistance of navigating vessel would be cut down and lead to the purpose of improving the velocity or decreasing the power of main engine while maintaining the velocity, if the technology of reducing the friction by microbubbles jetted from the bottom of vessels, which makes the water close to the vessel's bottom become the mixture or water and microbubbles and form a thin gas layer, could be applied in ships.In this article, the author firstly analyzed the mechanism of reducing the friction resistance. After analyzing the instance of several forces of single microbubble, the author found the main factors of influencing friction resistance. Because the movement of microbubbles is always in the way of cluster and this movement is different from it of single microbubble, primary analysis of this content is carried in this article for knowing more about the main factors of friction resistance.In experiment study, the author designed two models-small scale modeland large scale model according to difference of experiment pool and analyzed the feasibility of the two models. As for the device producing the microbubble, considering the utility and innovation, the porous material made from silicon is adopted. Furthermore, a plan of designing the small model having a step bottom is provided in the experiment of restricting model. And a plan of jetting from foreside and middle part of the large scale model is adopted in experiment, all these are our innovative points.From above schemes, the experiment of reduced resistance has been done under the condition of different velocity of different velocity, different gas flux, waterline and so on. The conclusion from the experiment is that the ratio of reduced resistance is relevant with the velocity of water stream, gas flux, the waterline of ship model and the dashplate. When the gas flux is constant, the ratio of reduced resistance is improved with the increase of the water stream velocity. However, the relationship between the gas flux and the ratio of reduced resistance is more complex. When the water stream velocity is constant, if thegas flux is small in contrast, the effectiveness of reduced resistance is not obvious. But when the gas flux increased, the jetted gas will form "archform" which will augment the shape resistance. In this condition, resistance could not be reduced and could even be increased, if the gas flux continues to be increased. So that there needs setting a baffle called step, which introduces the jetted gas into the turbulence boundary layer. In this case, the shape resistance may be fairly great. Besides, the waterline of model has also some effect on the ratio of reduced resistance, which acts on the size of wave. If waterline is deep, the wave could not affect the deployment of microbubble layer, so there is almost no effect on ratio of reduced resistance. However, the wave would affect distributing of microbubble layer under the model so that the gas could leak out from the two sides of model and directly affect the ratio of reduce resistance if the waterline is very small. Whether installing the dashplate or not has also the great influence on the effectiveness the ratio of reduced resistance. From the results of experiment, conclusion is that adding dashplate will greatly improve the effectiveness of reduced resistance.The size of pole in the porous material is not considered in this article. This factor need further study in the future.